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So here is the jist of the problem. I have a 2000 plymouth neon, the other day during a trip the check battery light came on my dash. I was an hour from home riding at night so i turned off as many electronics as possible and rode it out. I made it home and the next day began to investigate. the car started up fine and it rode well. I went to autozone and had them test my battery and altenator. The battery was at 45% after the night before, but after being charged it tested fine. The diodes tested fine as well but the Voltage Regulator was shown as failing. On my neon, the Voltage Regulator is in the PCM rather than in the altenator like most cars. I am wondering where I can start to begin fixing this problem. my feeling is that the PCM overall is fine, otherwise I would have some other codes appearing on my dash or other symptoms. I also was told that the Voltage ragulator is partially controlled by a temperature gauge. On some neons its a battery temperature gauge but on my neon its the Intake Air gauge but failure of the intake air gauge would apprear as a check engine light and since I dont have a light it must be working. So either the voltage regulator is bad and the whole PCM needs replaced or the connections between the Voltage regulator and the altenator are bad. Does anyone have any ideas on how to test the voltage regulator in the pcm or how to fix the problem without a complete replacement?

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On that MY Neon an "intake air gauge" doesn't have anything to do with the charging circuit.

The amount of amperage produced by the generator is controlled by Electronic Voltage Regulator (EVR) circuitry within the PCM. EVR circuity is connected in series with the generator field driver terminal and ground. A Battery Temperature Sensor (BTS) is located on rear of front bumper beam on all models except Avenger, Neon and Sebring Coupe, and is used to sense battery temperature. On Avenger and Sebring Coupe, BTS is located within PCM. On Neon, BTS is located on bottom of battery tray. Sensed battery temperature and data from monitored line voltage is used by PCM to adjust battery charging rate. This is accomplished by cycling the ground path to control the strength of generator rotor magnetic field. PCM then compensates and regulates generator amperage output accordingly. The PCM monitors critical input to control fuel injection, ignition, emission and other engine management functions. The PCM is also programmed to monitor charging system related circuits.

You need to test the whole charging circuit starting at the alternator since that's where everything comes together.

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The feed wire is a B+ source coming off the ASD relay. The other terminal label is self explanatory and is a 12v PWM signal being pulled to ground. The B+ terminal goes from the generator to the starter with a fusible link inline and then to the battery. The ground will either be a ground strap from the engine to the firewall or to the battery or some combination.

Make sure your battery connections are clean and tight. Do a visual inspection and check to make sure all grounds are clean and tight.

Check for Charging voltage at the generator B+ terminal. Is it greater than the voltage at the battery B+ terminal? Then you probably have an open or excessive voltage drop between the two. Since the car starts than you can infer that the cable from the battery to starter is OK and that the problem lies in between the starter and generator.

Check for 12v at the generator feed terminal is there 12V? If not then gain access to the PCM and test for 12v on connector 1 terminal 6 (dark green/orange), Is there 12V? If yes than you have an open or excessive voltage drop on the feed circuit and you need to trace the wiring from the ASD relay to the generator.

Check for a PWM signal on the PCM/Field terminal. IIRC you can check this with a multimeter, but it's best to check with a scope. If you're using a multimeter you should probably expect to see around 6v. If you get nothing at all or it shows an open circuit then check for continuity between the PCM connector 1 terminal 8 and the PCM terminal on the generator. If the wiring is OK replace the PCM.

You can pretty much ignore anything to do with the battery temperature sensor as the charging system will work fine without it and wouldn't illuminate the MIL even if there were a problem with it despite how service information describes the circuit.

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